This invention relates to a method for producing coated thermoplastic substrates that have smooth upper and lower surfaces with certain critical criteria which are particularly useful in optical media applications, such as CD-ROM cards. Such applications require the use of substrates having polished surfaces on both sides, a birefringence of less than twenty-five nanometers (25 nm) and wherein the coating and thermoplastic substrates have a low delta refractive index. Further requirements for optical media applications include abrasion, chemical and UV radiation resistance, and clarity. Moreover, thermoplastic films must transmit, rather than reflect, light to be acceptable for optical applications.
Polycarbonate films generally have acceptable levels of clarity and strength, but lack acceptable levels of birefringence, abrasion resistance, chemical and UV radiation resistance, and smoothness for optical applications. For example, uncoated polycarbonate films generally have high birefringence, which is in part due to the levels of stress which are placed on the film during its formation. This stress is frozen into the film upon cooling. Moreover, uncoated polycarbonate films generally have poor abrasion resistance and chemical resistance. Uncoated polycarbonate films are also susceptible to degradation by UV radiation.
Radiation curable acrylic coatings and methods for their application to polycarbonate substrates are known. (See, e.g., European Patent No. 228,671). While conventional methods exist for applying radiation curable acrylic coatings to polycarbonate film, the adhesion of these cured coatings to the underlying polycarbonate can be less than desirable. Moreover, conventional methods do not carefully control the smoothness of the films, which is critical for optical applications. For these reasons, polycarbonate films have not been compatible with optical applications because the required adhesion, clarity and smoothness properties have not generally been attainable prior to the present invention.
Conventional radiation curable acrylic coating compositions are also problematic because they employ non-reactive solvents to reduce the viscosity of the coating compositions during application thereof to the polycarbonate substrate. These non-reactive volatile components must later be eliminated from the coatings by applying a forced hot air drying system, which may produce a coating having unacceptable bubbles and surface roughness. It is also undesirable to use coating compositions containing substantial levels of non-reactive, volatile components such as solvents, because they create environmental and safety concerns.
Abrasion resistant thermoplastic films having improved optical properties may be manufactured by curing a solvent-free radiation curable coating composition after application to the surface of a polymeric sheet or film. This curing process may be accomplished by directing radiant energy through the substrate opposite the surface being coated. This “cold-casting technique” is further illustrated in U.S. Pat. No. 5,468,542, to Crouch, which is hereby incorporated by reference. Although improved surface characteristics can be achieved with the radiation curable coating compositions disclosed in U.S. Pat. No. 5,468,542, such coated thermoplastic films are not suitable for optical applications because no effort is made to match the indices of refraction of the coating and substrate to avoid light reflection at the interface (i.e., provide a low delta refractive index).
Also, U.S. Pat. Nos. 5,455,105 and 5,271,968 to Coyle et al. disclose an article and process for producing acrylic coated polycarbonate articles particularly films. While the process disclosed therein is suitable for producing a coated thermoplastic film, the references do not disclose or address the requirement for producing a film that has a critical properties of low birefringence and a minimal or low delta refractive index between the coating applied to the film substrate and the film substrate. These patents are hereby incorporated by reference.
In summary, conventional methods are inadequate for producing thermoplastic film suitable for use in optical media applications, because such methods do not produce thermoplastic film that has acceptable levels of birefringence, abrasion resistance, chemical and UV resistance, smoothness, and clarity together with a low delta refractive index (i.e., the difference in refractive index between the coating and the substrate). Specifically, birefringence, which is related to the residual stress in the film upon molding and cooling, must preferably be kept below 25 nm. Also, the film must be polished on both sides to avoid light scattering. Moreover, if the film is coated, the delta refractive index must be sufficiently low to avoid unacceptable light reflection at the interface.
It is also generally known in theory to produce polycarbonate films by extruding heated resin and passing the extruded resin through a nip between two polished metal rolls. This process is unsuitable for producing low birefringence films because it introduces a high level of stress in the films which increases the birefringence of said films.
In view of the foregoing, it would be advantageous to provide thermoplastic films which are suitable for optical media applications. It would further be desirable to provide an efficient method for making such films which avoids the shortcomings of the prior art.
Accordingly, it is one object of this invention to provide a particular thermoplastic film substrate suitable for coating with a particular coating composition wherein one side is textured and the other side is a polished surface, the film substrate having a low residual stress that may be subsequently coated with a particular coating composition and thereby forming a film having two smooth sides.
It is another object of the invention to provide coated thermoplastic films wherein the top and bottom surfaces are sufficiently smooth to avoid unacceptable light scattering, and the delta refractive index between the coating and film is sufficiently low to avoid unacceptable light reflection at the interface.
It is yet another object of this invention to provide a method for producing coated and uncoated thermoplastic films, which method does not require using solvents or other volatiles which may lead to imperfections in the films and present environmental concerns.
Still another object of the invention to provide a method of making a coated film having low birefringence.
It is another object of the invention to provide a method of making a one-side textured and one-side polished film substrate having low residual stress.
It is a further object of the invention to provide a method of making a coated thermoplastic film which is compatible with optical applications wherein the coating adheres strongly to the substrate.